Turning The Tables On Shaft Voltage And Bearing Currents

High bearing current concentrations can damage bearing raceways. This damage can degrade bearing performance and even shorten motor life. Why would some AC motors powered by adjustable frequency drives (AFDs) experience bearing failures after only a few months of operation? Is roughness on bearing raceway surfaces a symptom? A recent examination of problematic AC motors operating in a clean room addressed

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High bearing current concentrations can damage bearing raceways. This damage can degrade bearing performance and even shorten motor life.

Why would some AC motors powered by adjustable frequency drives (AFDs) experience bearing failures after only a few months of operation? Is roughness on bearing raceway surfaces a symptom? A recent examination of problematic AC motors operating in a clean room addressed these items, among others. The source of this damage is, in part, due to pulse width modulated (PWM) drive-induced voltage. Let's see why this happens and what manufacturers are doing about it.

High bearing current concentrations can damage bearing raceways. This damage can degrade bearing performance and shorten motor life. In this case, electric discharge machining (EDM) damages the bearing raceway. High bearing current discharge (focused in one spot) causes microscopic pitting of the raceway. Continued exposure to bearing current and high shaft voltages softens the raceway metal. This creates fluting (transverse grooves) when the ball bearings roll over the pits. The result: increased mechanical wear and premature bearing failure.

Both AC and DC motors develop bearing currents, regardless of horsepower. This current exists whether a motor runs across-the-line or from an AFD.

Basically, as a motor's speed increases, its bearings ride on a lubricating oil film. That film (only 0.2 to 2 microns thick) forms a boundary between the bearing race and the ball. The exception to this boundary is the existence of instantaneous asperity point contacts. These microscopic peaks and valleys appear in the bearing surface finish. The oil film forms a capacitor that's charged by the shaft voltage. Under ideal conditions, high rotor shaft voltages will not charge, due to asperity contact. Without contact, your motor will have these rotor shaft voltages.

What if this doesn't happen? Sometimes, the rotor-to-ground voltage (across the oil film capacitor) exceeds a limit. That limit is the dielectric breakdown voltage the film can withstand.

Other times, a ball bearing asperity point makes contact with the raceway. Let one of these events occur, and a destructive instantaneous high discharge current attacks your bearings. The result is a pitted bearing or raceway, which can lead to mechanical failure.

We normally associate this phenomenon with larger horsepower AC motors. Such motors generate sufficient magnitudes of end-to-end, rotor-to-ground voltage to charge the oil film capacitor to break down voltages.

Small motors also experience bearing current damage. This can happen when you power them with PWM voltage source inverters that have steep-fronted waveforms. As more AC motor installations apply AFDs with PWM inverters, this is becoming a problem-one that's not as rare as you might believe.

PWM inverters may cause excessive shaft voltage because they excite an electrostatic capacitive coupling effect between the stator and rotor. This creates rotor-to-ground voltage. Motors running across the line may see stator neutral-to-ground voltage of about 60V peak. In contrast, AFD-driven (PWM) motors may see stator neutral-to-ground voltages of hundreds of volts.

Maximum stator neutral-to-ground voltage is a zero sequence source of approximately the drive's DC bus voltage divided by two. (Write this as Vbus/2). But that's not the limit. One way you get a modulated stator neutral-to-ground voltage is through system components-for example, common-mode chokes. Another way to get it is to install a long cable between your drive and motor. Obviously, your first line of defense is to keep that cable as short as possible. Often, this isn't very short.

Let's not forget capacitance inside the motor. Stator-to-rotor capacitance is the main coupling mechanism for rotor shaft voltage. We don't want rotor shaft voltage. Stator-to-rotor capacitance is a low impedance at the PWM frequencies. It allows therotor to charge and discharge through the bearing oil film capacitor (when riding the film). Because you can't rely on the oil to give you sufficient withstand voltage, you have EDM discharge (especially when you don't get enough bearing contact). This happens particularly at bearing asperity contact points, which are microscopic peaks and valleys appearing on bearing surfaces.

Should you avoid PWM inverters? Certainly not! Most AFD applications probably don't have bearing current problems. AFD-induced bearing current may only be a problem in light shaft-load applications like clean rooms and HVAC installations. These kinds of applications place minimal shaft radial load on the motor. They don't give you additional bearing paths to ground. Typically, these lighter loads provide less bearing contact area to dissipate the heat generated by bearing current. Further, they create higher current density levels in the raceway.

We can classify possible solutions as drive-oriented or motor-oriented. Overall, motor-oriented remedies are the most reliable because the side effects of drive adjustments tend to offset effects of reducing or eliminating shaft voltage.

There are two promising motor-oriented solutions: installing a shaft grounding attachment on the rotor, and specifying an Electrostatic Shielded Induction Motor (ESIM).

A shaft grounding attachment on the rotor attempts to maintain rotor-to-ground voltage at frame potential, by dragging metallic-impregnated brushes onto the shaft to bleed off excess voltage buildup. Yes, you have increased purchase and maintenance costs. On the other hand, grounding brushes are a viable short-term solution.

An ESIM is probably the best long-term motor-oriented solution. It re-routes stator winding coupling capacitance current to a grounded, internal, non-rotating shield incorporated in the motor air gap. This design maintains rotor-to-ground voltage near zero, independent of the stator neutral modulation voltage. Since the AC motor transfers power by electromagnetic induction across the air gap, the single point grounded shield does not affect the power rating.

What about drive-oriented solutions? You can reduce the effects of bearing current by installing filters on the motor power circuit or adjusting the AC drive. You can use potential transformers or coupling L-C filters to control stator neutral voltage levels. In these approaches, common-mode signals may couple back to the line voltages with opposite phase, or these signals may return current to the inverter through a rectifier.

A disadvantage with these passive solutions is that each application requires tuning components for adequate attenuation of the common-mode voltage. That's because common-mode voltage is the source of the potential for shaft voltage. Output filter devices (R-L-C drive output filters, designed to reduce reflected wave line-to-line motor voltage) also tend to reduce dv/dt bearing currents. See the sidebar "How Bad Can It Get?" However, the inductances in these filters increase stator neutral modulation, due to resonance effects. Furthermore, they increase the potential for rotor shaft voltage, oil film breakdown, and EDM problems.

To find out if you have a potential problem, you should measure rotor-to-ground voltage with an oscilloscope. Connect the unit between motor frame ground and a multi-strand wire brushed up against the rotor shaft. Measurements greater than 2V peak may only indicate the possibility for bearing current. The only positive way to identify and separate bearing currents is with the test motor setup shown in Fig. 3, on page 62 of the original article). The grounding strap allows you to differentiate between destructive high values of EDM bearing currents and lower-level, less destructive "rate of change of voltage with respect to time" (dv/dt) bearing currents. The dv/dt current is a capacitive current induced when a power device switches in the PWM drive.

To determine a safe current density requires calculating bearing contact area-an onerous task at best. So there's not a good indicator available for the field-except to run the existing application and analyze problems as they occur.

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